Electrical machines, in particular large electrical machines such as those used in power station installations, should be monitored continuously and/or should be analyzed from time to time in order to ensure smooth operation, in order to identify fault states in good time, and in order to avoid uncontrolled failures.
There are various methods and apparatuses for this purpose for a large number of possible fault states, and these will be described briefly.
For example, U.S. Pat. No. 5,006,769 describes an apparatus for detection of turns shorts in the rotor winding of generators. In this case, the shaft voltage is measured via a specific grounding module, and is analyzed in a specific manner by means of Fourier transformation.
For its part, EP 0271678 proposes an apparatus which allows statements to be made about the suitability for use and the functional reliability of the shaft grounding and the insulation of the shaft. The apparatus proposed in this case, which is actually intended to reduce shaft voltages, allows statements to be made about the overall grounding conditions in the electrical machine.
DE 19742622 describes an apparatus which allows the identification of spark erosion on the shaft, (particularly in the bearings), as can occur when high voltages are present and which is generally associated with damaging effects, when measuring the shaft current and shaft voltage on the shaft.
A large number of documents describe apparatuses which are used for detection of brush sparking on the sliprings. These include, for example, U.S. Pat. No. 3,653,019, U.S. Pat. No. 4,058,804, U.S. Pat. No. 4,163,227 and U.S. Pat. No. 4,451,786. In this case, a measurement apparatus which must be provided specifically for this purpose is connected directly to the brushes and, and this is the major aspect of these documents, data processing is then carried out which makes it possible to distinguish between interference signals and useful signals. This is done primarily using the gating method in which interference signals are identified and the detection process is interrupted during the interference signals (blanking). Another method for detection of brush sparking is proposed in U.S. Pat. No. 4,577,151, in which a characteristic signal is detected via an antenna which must be provided specifically for this purpose, and is then supplied to an analysis unit. For this purpose, the antenna must be arranged in the vicinity of the sliprings in order to receive the corresponding radio-frequency signals. The signals are then amplified and demodulated in a specific analysis unit, and are then supplied to a detector.
One method for identification of a further fault state is described in U.S. Pat. No. 4,814,699. This relates to the detection of partial discharges in the stator, rotor and in the excitation apparatus. In this case, the shaft is used as an antenna and the radio-frequency pulses are output via one of the (insulated) shaft bearings or via a coupling coil which must be provided specifically for this purpose. The shaft bearings must for this purpose be electrically insulated from ground and must also be designed so as to make it possible to distinguish between interference signals in this specific signal and useful signals.
A further serious fault state which must likewise be monitored is oscillations in the shaft run of the machine, in particular in a turbogenerator run. These oscillations may be of a different nature. They may be bending oscillations or transverse vibration occurring in the direction at right angles to the shaft. Furthermore, so-called hunting may occur, that is to say quasi-periodically-damped changes in the rotation frequency of the shaft above the mains frequency of normally 50 Hz (or 60 Hz) and which are produced by, for example, sudden disturbances in the mains. The actual torsional oscillations which can be produced, for example, by sudden load increases on the mains are recognized as a third type of shaft oscillation. In this case, torsional oscillations are oscillations which manifest themselves in a non-standard rotational frequency along the shaft (phase shifts or frequency shifts), that is to say they produce torsion on the shaft.
Torsional oscillations are very small oscillations, normally in the region of 0.01 degrees phase amplitude, but which nevertheless can lead to a very major load on the shaft and, in particular, coincidence between the natural frequency of such a torsional oscillation and the stimulus exciting it can lead to dangerous resonant increases in such torsional oscillations, which can even result in the shaft fracturing. Torsional oscillations may in this case be at frequencies from a few Hz up to 200 Hz, with the frequency naturally depending on the material characteristic and thickness of the shaft, the masses which are connected to the shaft and the size of the installation. These torsional oscillations can become extremely critical, particularly in large installations with long shafts.
Owing to the importance of this problem, a large number of patent specifications have also already dealt with oscillations such as these. For example, U.S. Pat. No. 3,934,459 describes test equipment and a method for measurement of the torsional oscillations of the entire shaft run of a turbine/generator system. The torsional oscillations are recorded at one or more points on the shaft for this purpose by means of one or more sensors which are not specified in any more detail but must be provided specifically for this purpose. The further processing of the torsional signals is the main subject of this document. The further processing is carried out by filtering using bandpass filters and by means of multiplication operations in order, at the end, to determine the maximum torsional torques.
U.S. Pat. No. 3,885,420, U.S. Pat. No. 4,148,222, U.S. Pat. No. 4,137,780 and U.S. Pat. No. 4,317,371 should also be cited with reference to oscillations, all of which describe test equipment and/or methods for measurement of torsional oscillations in electrical installations such as these. Toothed wheels which are connected to the shaft and must be provided specifically for this purpose are used as signal transmitters for detection of the torsional oscillations, and produce electrical signals by means of sensors. The signals are processed further by means of bandpass filters and multiples, etc. U.S. Pat. No. 4,317,371 describes one specific demodulation method for measurement of phase shifts. The method comprises the production of relatively low intermediate frequencies followed by frequency demodulation, as is known per se from radio engineering. Another method (U.S. Pat. No. 3,885,420) uses a PPL (phase locked loop) for demodulation. U.S. Pat. No. 4,444,064 is also cited, which describes a method in which, first of all, a magnetic pattern must be additionally applied to the shaft, and is then used as a pulse transmitter. Another concept for measurement of torsional oscillations is based on using the voltage at the phase winding terminals of the generator (which uses permanent magnet excitation and is coupled to the shaft) in order to determine such oscillations. The torsional oscillations are deduced by evaluation of the voltages that are produced and are tapped off by means of appropriate terminals. The evaluation process is once again carried out by means of frequency demodulation (PPL technique). This arrangement has the advantage that its costs are lower than those of the toothed wheel solution.
Further fault states which may possibly occur and which should also be mentioned include, for example, defective insulation on the rotor winding as well as rotationally synchronous shattering marks and/or witness marks which should likewise be detected in order to prevent failures.